The present application relates to low-power integrated circuits which use power islands, and more particularly to the implementation of decoupling capacitance in such circuits.
Decoupling capacitors are often used in chip design to reduce noise during switching. In the logic areas filler cells which include capacitors connected between power and ground are typically used. As voltage scaling progresses, and devices become increasingly sensitive to transient overvoltages, control of switching noise becomes increasingly desirable.
However, a conflicting trend is the demand for lower off-state leakage. Decoupling capacitors are commonly implemented as simple accumulation capacitors. While the gate oxide provided by a deeply scaled process allows a very high specific capacitance, it also provides a relatively high leakage current density.
The leakage density of gate oxides is currently scaling unfavorably. Thus as processes continue to scale to smaller dimensions, the traditional approach to decoupling capacitors provides leakage current densities which are unacceptable.
Many portable electronic systems are critically limited by battery life. For example, users do not like heavy cell phones, but users also do not like their cell phones to run out of power and shut down. One way to improve battery life is to increase the energy efficiency of the electronics components in the portable system. Power consumption when a portable system is active is an important component of battery life, but another is power consumption when the system is in standby.
Power islands are a common technique in low-power design. Power islands help to reduce standby current by keeping powered only the circuit sections that are needed during sleep or standby operation.